Image reject mixer

ABSTRACT

Circuit  200  comprises an input amplifier stage  290 , phase-splitters  292, 293  and mixer cores  294, 295 . An input signal is applied to terminal  220 , a local oscillator signal applied to terminals  230, 231  and a 90° shifted local oscillator signal is applied to terminals  232, 233 . The In-phase differential signal is output at terminals  240, 241  and the quadrature signal is output at terminals  240, 233 . Phase-splitters  292, 293  comprise base-coupled transistors  202, 203, 205, 206  which are biased by a potential applied to terminal  262 . As these phase-splitters are driven by input amplifier stage  290 , which acts as a current source, the arrangement has very good noise properties. Degeneration inductor  280  reduces the noise figure of the circuit further because it is a noiseless component. Phase-splitters  292, 293  and mixer cores  294, 295  are preferably cross-coupled to allow cancellation in phase-splitters  292, 293  of the second harmonic of the local oscillator signal generated at the inputs to mixer cores  294, 295.

BACKGROUND OF THE INVENTION

The present invention relates to image reject mixer circuits and inparticular to image reject mixer circuits having a single ended inputand two differential outputs. Radio frequency image reject mixers arevery popular blocks of modern radio systems and are often used inpreference to superheterodyne receivers, especially where frequencyagility is required. The parameters of an image reject mixer determinethe main characteristics of the system into which they are incorporated.

Prior art image reject mixers are generally based on the Gilbert cell orthe micromixer circuit configuration. In the case of the Gilbert cell,an image reject mixer is compiled simply by connecting two Gilbert cellcircuits in parallel. The input signal is split into two branches wherethey are mixed with an unshifted local oscillator signal and a 90°shifted local oscillator signal respectively.

A section of an image reject mixer circuit based on the micromixerconfiguration is shown in FIG. 1.

In use, an input signal is applied to terminal 120, a local oscillatorsignal is applied to terminals 130, 131 and a 90° phase shifted localoscillator signal is applied to terminals 132, 133. A referencepotential is applied to terminal 125 to bias transistors 103 and 104. Byvirtue of resistors 150, 151, transistor 105 and biased transistor 103,a signal input at terminal 120 will give rise to complementary outputcurrent signals at the collector electrodes of transistors 103 and 101.Mixer core 160, formed by transistors 106-109, mixes the current signalsfrom the collector electrodes of transistors 101 and 103 with the localoscillator signal applied to terminals 130, 131 and outputs a currentsignal at terminals 140, 141. Transistors 102 and 104 produce at theircollector electrodes substantially the same current signal as isproduced by corresponding transistor 101, 103 because thesecorresponding transistors are driven by the same input signal. Mixercore 170 mixes the current signal from the collector electrodes oftransistors 102, 104 with a 90° phase shifted local oscillator signalapplied to terminals 132, 133 and outputs a current signal at terminals142, 143. Because an unphased oscillator signal is applied to terminals130, 131 and a 90° phase shifted signal is applied to terminals 132,133, output terminals 140, 141 will show an In-phase differential outputand output terminals 142, 143 will show a Quadrature differentialoutput.

As will be appreciated, the mixer circuit shown in FIG. 1 is incomplete.The full mixer circuit implementation would also have means forphase-shifting the output of mixer core 170 by 90° and summing theresultant signal with the output from mixer core 160. This would resultin either the image band signal or the signal band signal as thecomplete mixer circuit output, depending on the sign of the 90°phase-shift imposed on the signal output from mixer core 170.

Whilst the image reject mixer circuit section of FIG. 1 has a widedynamic range and very linear operation, the presence of so manyresistors gives the mixer circuit very poor noise properties.

Image reject mixers constructed from Gilbert cell circuits have poornoise properties due to resistors in the main current paths, currentsources experiencing high frequency, large voltage swings and poortransistor arrangements. It is difficult also to design an image rejectmixer using Gilbert cell circuits so that it has a particular inputimpedance. This can be a drawback when impedance matching with a pre-ampstage is necessary. There exists a need for an image reject mixercircuit with improved noise

SUMMARY OF THE INVENTION properties.

In accordance with the present invention, there is provided an imagereject mixer circuit arrangement comprising an input amplifier connectedto first and second phase-splitters, the phase-splitters each having twosubstantially complementary outputs, a first mixer core arranged to mixtwo of said phase-splitter outputs with a local oscillator signal and asecond mixer core arranged to mix the other two of said phase splitteroutputs with a phase shifted local oscillator signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying drawings, of which:

FIG. 1 shows a section of a prior art image reject mixer circuit, and

FIG. 2 shows an image reject mixer circuit section in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 2, an image reject mixer circuit section 200 is showncomprising input stage 290, phase-splitter stages 292, 293 and mixercores 294, 295. Mixer cores 294, 295 are substantially the same as thoseused in prior art mixer circuits. The noise properties of image rejectmixer circuit section 200 are better than those of prior art mixersbecause of improved amplification and phase splitting stages 290, 292,293.

Terminal 220 forms the mixer circuit input terminal, terminals 240 and241 form the In-phase output terminals and terminals 242, 243 form theQuadrature output terminals. In use, a local oscillator signal isapplied to terminals 230, 231 and a 90° phase shifted oscillator signalis applied to terminals 232, 233. Potentials are applied to terminals260 and 261 to form bias potentials with resistors 253 and 252respectively whilst the potential applied to terminal 262 forms two biascurrents with resistors 251 and 254. Resistors 251 and 254 arepreferably high value resistors so as to deliver small but substantiallyconstant currents to phase-splitters 292, 293.

Input stage 290 is a transconductance amplifier. It receives a voltagesignal applied to input terminal 220 and supplies output current to bothphase splitters 292, 293. Transistor 201, of input stage 290, isconnected to each of transistors 202 and 205, of phase splitter stages292, 293 respectively, in a cascode configuration. Transistor 202 isbase-coupled to transistor 203 and, likewise, transistor 205 isbase-coupled to transistor 206. Degeneration inductor 280 connects theemitter electrode of transistor 201 to ground potential. Due to thecomplex nature of the common emitter current gain β of the transistor201, inductor 280 effects series negative feedback in the base-emittercircuit of transistor 201.

Transistors 202 and 203 have complementary collector currents.Variations in the collector current of transistor 201 directly causevariations in the distribution of the current flowing in resistor 251between the base electrodes, and hence the collector electrodes, oftransistors 202 and 203. Transistors 202 and 203 introduce a negligiblelevel of noise into mixer circuit 200 because they are driven by theoutput current of amplifier stage 290 rather than being voltage driven.Transistors 205 and 206 are configured in the same way as transistors202, 203.

Transistors 202 and 205 are current driven by transistor 201, resistor252 and a potential applied to terminal 261. Transistors 203 and 206 aresimilarly driven by transistor 204, resistor 253 and a potential appliedto terminal 260. The balancing of the output signals from eachphase-splitter 202, 203 and 205, 206 can thus be controlled by eitherchanging the values of resistors 252, 253 or by changing the potentialsapplied to terminals 260, 261. Capacitors 271, 275, 276 are simply acgrounding and dc blocking capacitors.

Cascode circuits per se are well known for their good noise properties.From FIG. 2 it can be seen that these good noise properties are achievedbecause transistors 202 and 205 prevent the collector of transistor 201from swinging and thereby substantially eliminate the Miller effect.

Inductor 280 is a noiseless component which provides substantiallyfrequency independent degeneration over a particular frequency range.This range is dependent on the value of inductor 280 and thebase-emitter resistance of transistor 201 at the desired frequency. Thevalue of inductor 280 also affects the gain of amplifier stage 290 andits linearity. Although a resistor could be used in place of inductor280, amplifier stage 290 has much more linear characteristics and betternoise properties when inductor 280 is used.

Inductor 280 can be implemented, in whole or in part, with the parasiticinductance of IC packaging, bonding wires and/or connecting pins.

Transistors that are used in low noise applications are generallyfabricated with large emitter areas. These transistors have a lowerbase-emitter resistance than a smaller area transistor and hence produceless noise.

Transistors 201-203, 205-206 are preferably implemented as large emitterarea transistors and more preferably with transistor 201 having a largerarea than transistors 202-203 and 205-206. The sizing of transistor 201is particularly important because it determines a number of propertiesof the amplifier stage 290. A larger area transistor will have betternoise properties because the input impedance of the transistor will belower. However, a larger area transistor will also have higher parasiticcapacitances, and hence leakage, and a lower current gain β caused by alower current density.

The size of the current flowing in transistor 201 affects its impedanceand also therefore the properties of amplifier stage 290. Operatingtransistor 201 with a low current will give it good noise properties butwill also cause β, and hence overall gain, to be lower than it would befor a higher operating current.

A trade-off needs to be made between noise figure and gain when choosingwhat transistor area and what driving current should be incorporatedinto a particular image reject mixer circuit design.

At high frequencies, the behaviour of transistors 202, 203, 205 and 206will change because of the parasitic capacitances present across boththe base-emitter and base-collector junctions of these transistors.Leakage will occur between the emitter and the base electrodes oftransistors 202 and 205 causing unbalancing of the outputs of each phasesplitter 292, 293.

This can be compensated for by forming transistors 202 and 205 withlarger emitter areas than those of transistors 203, 206. These largerarea transistors will have a lower current density and hence a lower β.This will cause extra current to flow in the base of transistors 202,205 to compensate for current lost in the parasitic capacitances.

The emitter areas of transistors 202, 203, 205, 206 required to balancethe output signals will depend on the frequency at which mixer circuitsection 200 is to be operated, because β is frequency dependent, and onthe currents driving these transistors. Mixer core 294 receives asubstantially sinusoidal local oscillator signal as a differentialvoltage signal on terminals 230, 231. When the voltage on terminal 230is positive, the voltage on terminal 231 will be negative causingtransistors 207 and 210 to be switched on and transistors 208 and 209 tobe switched off. The collector current of transistor 202 will thereforebe routed to output terminal 240 whilst the collector current oftransistor 206 will be routed to output terminal 241. The collectorcurrents of transistors 202, 206 will obviously be routed to theopposite terminal 240, 241 when terminal 231 receives a higher voltagethan terminal 230.

Mixer core 295 operates in substantially the same way, routing thecollector currents of transistors 203, 205 alternately to outputterminals 242 and 243. Transistors 211-214 are switched, in use, undercontrol of a 90° shifted local oscillator received on terminals 232,233.

Capacitors 273, 278 serve as filters of the second harmonic of the localoscillator signal, applied to terminals 230, 231, that would normally begenerated at the inputs of mixer core 294. This harmonic is generatedbecause of the difference in the switching-on and switching-off delaysof transistors 207-210 in mixer core 294. This harmonic would normallybe mixed, as well as the input current signals from the collectorelectrodes of transistors 202, 206, by mixer core 294 and produce aparasitic signal at the local oscillator frequency at output terminals240, 241. Capacitors 274, 277 similarly serve as filters of the harmonicsignal generated by transistors 211-214 in mixer core 295. The values ofcapacitors 273-274, 277-278 have to be chosen as a trade-off between theefficiency of filtering and the efficiency of the conversion of thesignals input to mixer cores 294, 295.

Additionally, a large proportion of the second harmonic is cancelled atthe base electrodes of transistors 202-203, 205-206 by virtue of thecross coupling of phase-splitters 292, 293 and mixer cores 294, 295because the second harmonics produced at the inputs to mixer core 295are 180° out of phase to those produced at mixer core 294.

However, it will be obvious to the skilled man that this cross-couplingis not an essential part of the invention. The second harmonic of thelocal oscillator would be cancelled, though not as much, by capacitors273, 274, 277, 278 even if the collector electrode of transistor 203 wasconnected to mixer core 294 and the collector electrode of transistor206 was connected to mixer core 295.

Resistor 250 and capacitor 272 are preferably incorporated into an imagereject mixer circuit design to improve the linearity of the mixer and toallow its input impedance to be tuned.

Resistor 250 is connected between transistors 201 and 202 to create apotential at the collector electrode of transistor 201 and 202 from thecurrent flowing there. Resistor 250 will be of low value, say 20-30 Ω,so not introducing much noise into the circuit. Inductor 280 causes thevoltage at the emitter electrode of transistor 201 to lead the voltageat the base electrode by 90°. Negative feedback is thus achievable byconnecting capacitor 272 across the base and collector electrodes oftransistor 201. This feedback will help to minimise the noise created bytransistor 201 and improve the overall noise figure of mixer circuitsection 200. Mixer circuit section 200 will also have improved linearitycharacteristics.

Capacitor 272 and resistor 250 will also have an effect on the inputimpedance of transistor 201, and hence mixer circuit section 200,thereby allowing the impedance to be tuned in the design of the mixercircuit. It is even possible to make the input impedance purely real.

Although the embodiments have been described solely with regard to npnbipolar resistors, the invention is not restricted to such and couldequally be effected with pnp bipolar transistors or with field effecttransistors. The collector and emitter electrodes referred to will beinterchangeable with emitter and collector, source and drain or drainand source electrodes as the first and second main electrodes of a pnpor a field effect transistor.

What is claimed is:
 1. An image reject mixer circuit arrangement,comprising: a) an input amplifier having a single-ended input and asingle-ended output; b) first and second mixer cores each havingrespective first and second inputs; and c) first and second current modephase-splitters, each of the phase-splitters having a respective inputand two substantially complementary outputs, the inputs of both of thephase-splitters being connected to the output of the input amplifier,the outputs of the phase-splitters being arranged to carry signals atsubstantially the same frequency as the signals at the output of theinput amplifier, the inputs of the first mixer core being connected totwo of the outputs of the phase-splitters, the first mixer core beingarranged to mix signals provided at these phase-splitter outputs with anoscillatory signal, the inputs of the second mixer core being connectedto the other two of the phase-splitter outputs, the second mixer corebeing arranged to mix signals provided at these other phase-splitteroutputs with a phase-shifted version of the oscillatory signal.
 2. Theimage reject mixer circuit arrangement in accordance with claim 1, inwhich the outputs of the first phase-splitter and the outputs of thesecond phase-splitter are cross-coupled with the inputs of the first andsecond mixer cores.
 3. The image reject mixer circuit arrangement inaccordance with claim 1, in which the first phase-splitter comprisesfirst and second transistors having control electrodes connectedtogether and to a first current source, and in which the secondphase-splitter comprises third and fourth transistors having controlelectrodes connected together and to a second current source.
 4. Theimage reject mixer circuit arrangement in accordance with claim 3, inwhich the input amplifier comprises an input transistor having a controlelectrode arranged to receive an input signal and a first main electrodeconnected to the output of the input amplifier.
 5. The image rejectmixer circuit arrangement in accordance with claim 4, in which thecontrol electrode of the input transistor is connected via a firstresistor to a fist bias potential, and receives an input signal througha dc blocking capacitor.
 6. The image reject mixer circuit arrangementin accordance with claim 4, in which an inductor is connected between asecond main electrode of the input transistor and ground potential. 7.The image reject mixer circuit arrangement in accordance with claim 4,in which the first main electrode of the input transistor is connectedto a second main electrode of the first transistor and to a second mainelectrode of the third transistor.
 8. The image reject mixer circuitarrangement in accordance with claim 4, in which second main electrodesof both the second and the fourth transistors are each connected to arespective current source.
 9. The image reject mixer circuit arrangementin accordance with claim 4, in which the second and the fourthtransistors have second main electrodes connected to a common currentsource.
 10. The image reject mixer circuit arrangement in accordancewith claim 9, in which the second main electrodes of the second and thefourth transistors are connected to a first main electrode of a fifthtransistor, the fifth transistor having a control electrode connected toa second bias potential via a second resistor, and a second mainelectrode connected to ground potential.
 11. The image reject mixercircuit arrangement in accordance with claim 4, in which the outputs ofthe first and second phase-splitters are each connected to groundpotential by a respective filtering capacitor.
 12. The image rejectmixer circuit arrangement in accordance with claim 10, in which an acgrounding capacitor is connected between the first main electrode of thefifth transistor and ground potential.
 13. The image reject mixercircuit arrangement in accordance with claim 7, in which the first mainelectrode of the input transistor is connected to the inputs of thefirst and second phase-splitters by a feedback resistor, and the controlelectrode and the first main electrode of the input transistor areconnected together by a feedback capacitor.